The present invention relates to apparatus for leading a web threading tail over an empty space, which empty space is formed between two surfaces and the apparatus is arranged between the surfaces to lead the web threading tail from the surface preceding the empty space, over the empty space to the surface following the empty space, which apparatus includes a belt loop permeable by air, which is arranged to rotate in the direction of travel of the web threading tail and is supported by turnover rolls, and within which belt loop at least one foil strip is arranged to create a vacuum effect in the part of the belt loop that transports the web threading tail.
Empty spaces according to the introduction occur particularly in the dryer sections of twin-wire paper and board machines. The dryer section includes one or more cylinder groups, with drying cylinders set on two levels. In order to lead the web through the cylinders, each level has a separate wire running through it. This arrangement is termed a twin-wire dryer section. The web is guided by means of turnover rolls placed between the drying cylinders, in such a way that both the web and the wire travel for the longest possible distance on the surface of the drying cylinders. In practice, the web is lead over alternate drying cylinders on the different levels. Thus the web travels unsupported from one level to the other, over the empty space between the two levels. There are also similar empty spaces elsewhere in paper machines, for example, in the press section.
During normal operation, an empty space has little effect on the travel of the web in a twin-wire dryer section. However, when starting up production, the web is taken through the dryer section with the aid of a web threading tail cut from the web. This web threading tail that runs between the drying cylinder and the wire tends to catch on the surface of the drying cylinder, though it should move over the empty space into the closing throat formed by a drying cylinder and the wire on the other level. Because of this, special apparatuses are used to carry the web threading tail over the empty space from the opening throat to the closing throat.
Usually the web threading tail is carried by ropeways, but nowadays these are being removed, due to their unreliable and even dangerous operation. Cable-less apparatuses have therefore been developed, in which the web threading tail is lead over empty spaces with the aid of various guide plates with associated air jets. In addition, particularly in dryer sections, doctors are used to transfer the web threading tail from the drying cylinder to the guide plate. In the doctor, a blade or an air jet can be used. The doctors, guide plates, and air jets can also be separate.
The doctoring result achieved using an air jet is generally poor, which leads to breaks when leading the web threading tail. Frequently, attempts are made to improve the doctoring result by increasing the power of the air jet, but the jet then often interferes with the movement of the web threading tail. As air jets have mainly only a guiding effect, they cannot be used to tension the web threading tail. In addition, the nozzles used to create the jet are sensitive to variations in conditions and can only be imprecisely controlled. Doctor blades in contact with the surface of the drying cylinder wear in use and have a complex construction. In practice, threading must be assisted with manually-operated jet pipes, or even by hand, which substantially impairs work safety.
For long empty spaces, apparatuses based on a belt loop have also been developed. In it, the web threading tail, which is lead onto the belt loop, is propelled forwards by the rotation of the belt loop and a vacuum arranged inside it. WO publication number 0019013 discloses such an apparatus. However, the known devices are long, making it impossible to apply them, for example, to empty spaces in a dryer section. In addition, the apparatus requires special devices to detach the web threading tail and guide it on to the belt loop.